Signode Corporation, the assignee of the entire interest of the present invention, has heretofore developed several machines for feeding strap in a chute to form a loop around an article to be strapped and for tensioning the loop tight about the article. Typically, these machines also apply a seal to the tensioned loop or otherwise form a connection between the overlapping strap segments in the loop, and then sever the tensioned and sealed loop from the trailing length of strap.
The apparatus of the present invention relates to such strapping machines and is adapted to form a part of a strapping machine. Specifically, the apparatus of the present invention is intended to be used in a machine having a strap chute and a strap end gripping and sealing unit. The apparatus of the present invention feeds the strap into the chute and in a loop about a package. Subsequently, it first draws the loop tight about the package and then applies a high tension, during the application of which, the strap end gripping and sealing mechanism connects the overlapped ends of the strap loop.
Typically, after strap is fed into a strap chute around the package or article to be tied, and after the leading end of the strap is gripped by an appropriate strap end gripping mechanism, the strap loop is drawn tight around the article or package to a certain predetermined tension. This is referred to as "pre-tension". It is desirable, from the standpoint of strapping packages as quickly as possible, to perform the strap feeding and pre-tensioning operations as rapidly as possible. Thus, it would be desirable to provide a means for rapidly feeding and withdrawing the strap from the strap chute to tighten the loop about the package.
Though it is generally desired to perform all strapping operations as rapidly as possible, including the above-discussed feeding and "pre-tensioning" steps, the step of applying the final, high tension to the strap loop (before the overlapping strap ends are connected), is best performed relatively slowly. With many types of articles or packages which are bound by a loop of tensioned strap, the article or package may compress relatively slowly in response to a suddenly applied strap loop tension. The compression that continues to occur for some time after the sudden application of high tension will cause a subsequent reduction in the effective strap loop tension. Further, with many types of packages, the package and/or strapping machine tend to move relative to each other as the high tension is being applied. Thus, if the high tension is applied suddenly, proper securement of the package may not be achieved because of the possibility of post-tension compression of the package and because of the possibility that any necessary relative movement between the package and the strapping machine is not properly accommodated.
Additionally, the application of high tension at a high rate requires a great amount of power. High power requirements greatly increase the cost, size, and weight of strapping machines with few or no compensating benefits.
In order to feed the strap relatively rapidly into the strap chute and in order to rapidly draw the strap loop tight about the package during the "pre-tension" sequence, yet execute the high tension sequence relatively slowly, strapping machines in the past have been designed a number of different ways. Some machines have used a separate means (such as a hydraulic motor) to feed and/or pretension the loop and have used another separate means (such as an electric or hydraulic motor or pneumatic cylinder operator) to draw the high tension. Other machines have used transmissions that shift into a low gear for applying high tension, but these require an external control system or signal (e.g., traction wheel air motor back up pressure or strap switch release from a holding gate). It would be desirable to provide a strapping apparatus in which a single motor or drive means could be used to effect the feeding, pretensioning, and high tension sequences without the need for external controls or signals to shift from a high speed, low torque mode to a low speed, high torque mode.
Some strapping machines have been developed wherein a single motor is used to retract the trailing strap and pretension the strap about an article at high speed and to subsequently apply the high tension at the same high speed. However, high speed application of the final, high tension, as performed by these machines, is not always desirable for the reasons discussed above. Thus, it would be desirable to provide a strapping apparatus wherein a single drive means or motor could be used to apply the pretension relatively rapidly and to apply the high tension relatively slowly.
It would further be desirable to provide an electric motor to apply both the rapid pretension and the slow high tension instead of non-electric motor means (such as hydraulic or pneumatic actuators) to avoid having to supply hydraulic fluid under pressure or compressed air to the apparatus.
It would also be beneficial to provide a single, relatively low power electric motor for applying a rapid pretension and a slow high tension, without the requirement for a more costly, large, variable speed motor.
It would be advantageous to provide a single electric motor for applying the high tension to a strap loop, which motor would be small enough, when coupled with any necessary gear transmission mechanisms, to permit relatively efficient operation off of ordinary electrical lighting circuits as opposed to 3 phase, 220 volt or greater AC circuits.
Another salient feature would include means for feeding the strap by rotating members or traction wheels in direct contact with the strap, which wheels would not bite into, or otherwise damage, the strap. Prevention of scratches or other damage to the strap is very important since damaged strap can fail under tension and/or take on a "camber" which causes binding in the strap guide. Further, with metal strap, surface damage can provide a starting point for rust.
Further, it would be helpful if such traction wheels would slip when a predetermined, high torque was being transmitted to the wheels to prevent damage to the apparatus.
It would also be of some utility to provide traction wheels for feeding the strap, which wheels would have good wear characteristics and which, if biased together during periods of machine shut-down, would not deform and develop permanently set flat spots.
In connection with using a single electric motor, it would be very desirable to provide a power transfer or gear drive assembly means with two speed dual output capability for automatically transferring the motor power from the traction wheels at high speed during pretensioning to a suitable high tensioning assembly for applying the final high tension at low speed, without the need for complicated controls, by the automatic direct sensing of strap tension.
A further salutary effect would be realized by providing a strap feeding and tensioning apparatus that was relatively self-contained and relative lightweight and small so as to readily adaptable to many different packaging requirements and so that it could be easily and quickly replaced on site.